A sensorless DC motor does not have a position sensor for detecting a position of a rotor and can be downsized. Therefore, the sensorless DC motor is widely used for vehicle installations and home electric appliances.
The sensorless DC motor is, for example, driven by a PWM (Pulse Width Modulation) signal. A controller of the sensorless DC motor has a function for detecting a driving current. When the driving current is over a threshold, the controller determines to be an over current and lowers a duty ratio of the PWM signal. Because of this, the controller can prevent the sensorless motor from being burned-out.
Generally, because the threshold for detecting the over current is constant, there is a likelihood that the sensorless DC motor has following problems on starting the sensorless DC motor. For example, when driving the sensorless DC motor having a small torque characteristic with heavy inertia, it is necessary to generate a large driving current on starting the sensorless DC motor. However, if the threshold for detecting the over current is set low in order to prevent the sensorless DC motor from being burned-out, it may take a long time for the sensorless DC motor to follow up a forced commutation frequency on starting the sensorless DC motor. Contrarily, when driving the sensorless DC motor having a large torque characteristic with light inertia, if the threshold for detecting the over current is set high in order to prevent the over current from being detected more than necessary, a large driving current may be generated on starting, and an over drive where a rotation frequency of the rotor much exceeds the forced commutation frequency may occur.
JP-A No. 2001-275392 (Kokai) (hereinafter, “Patent Document 1”) discloses a technique for driving the sensorless DC motor so as not to exceed the threshold for detecting the over current on starting. However, by the technique of Patent Document 1, when driving the sensorless DC motor having a small torque characteristic with heavy inertia, it is difficult to set the sensorless DC motor to be a steady state quickly.
JP-A No. 4-317587 (Kokai) (hereinafter, “Patent Document 2”) discloses a technique to shift a self starting frequency, which is a frequency on starting the sensorless DC motor”, from high frequency to low frequency. However, by the technique of Patent Document 2, because the self starting frequency right after starting is high, the over drive may occur.